1. Field of the Invention
This invention relates to the cancellation and calibration procedures for removal of disturbances in magnetic disk storage systems.
2. Prior Art
Repeatable runouts (RRO) in magnetic disk drives are caused by a number of items, such as inaccuracy in disk assembly, unbalanced platters, spindle motor shaft tilt, servo writer noise and disk slippage after a shock, and so on. Especially in PERM (Pre-Embossed Rigid Magnetic) disks, since the tracks, including servo sectors, are pre-embossed and assembled into the drive there are runouts mainly due to the inherent eccentricity of the tracks. Typically, RRO are harmonic disturbances synchronized by rotation of a disk. As the track density (or tracks per inch, TPI) of a disk increases, this harmonic disturbance, that is, RRO, will become a dominant factor in track following errors, since runouts are not scaleable with the track density.
High-capacity disk drives have been tested with an RRO compensator based on the discrete Fourier transform (DFT) method Hobson, D., "Feed Forward with Seeks on Kiowa 2|," Control Systems Database (Seagate), Mar. 29, 1995, and Hobson, D., "Feed Forward/RRO Compensation Successful on Hawk 2 XL," Control Systems Database (Seagate), Dec. 22, 1995!. The need for an RRO compensator was especially keen for dedicated-servo drives. Although RRO are less severe in an embedded servo (or sector servo) drive, a RRO compensator is required for a multi-disk high TPI drive. RRO compensation based on DFT requires at least one full revolution of a disk before compensation becomes effective. Hence this technique is not suitable for a drive with large RRO such as a removable drive.
After various RRO cancellation algorithms were tested, the AFC (Adaptive Feedforward Cancellation) algorithm was proved to be the most effective and efficient for disk drive applications. AFC was proposed and tested for disk drive applications by various researchers at Carnegie Mellon University (CMU) and University of Utah (notably by Professor Marc Bodson). The results are published in Sacks et al., "Experimental Results of Adaptive Periodic Disturbance Cancellation in a High Performance Magnetic Disk Drive," Proceedings of the American Control Conference, San Francisco, Calif., pp. 686-690, June 1993; Messner et al., "Design of Adaptive feedforward Controllers Using Internal Model Equivalence," Proceedings of the American Control Conference, Baltimore, Md., pp. 1619-1623, June 1994; Bodson et al. "Harmonic Generation in Adaptive Feedforward Cancellation Schemes," IEEE Transactions on Automatic Control, Vol. 39, No. 9, pp. 1939-1994, September 1994; and Sacks et al., "Advanced Method for Repeatable Runout Compensation", IEEE Transactions on Magnetics, Vol. 31, No. 2, pp. 1031-1036, March 1995. Although not specifically demonstrated, it is believed that their algorithms are derived from adaptive control theory, see, for example, a classical textbook written by Sastry, S. and Bodson, M., "Adaptive Control: Stability, Convergence and Robustness," Englewood Cliffs, N.J., Prentice-Hall, 1987.
It turned out that AFC was equivalent to the Least Mean Square (LMS) adaptation algorithm developed for adaptive filters Widow, B. and Stearns, S. D., "Adaptive Signal Processing," Englewood Cliffs, N.J., Prentice-Hall, 1985; and Haykin, S., "Adaptive Filter Theory," Second Edition, Englewood Cliffs, N.J., Prentice-Hall, 1991!. Surprisingly the AFC researchers (at CMU and Utah) never mentioned about this link. LMS algorithms have been used in digital signal processing for a long time. They are especially effective for identification and cancellation of sinusoidal interference (adaptive noise cancellation problem). A similar algorithm based on LMS was patented by Workman, M. L, "Disk Drive File Servo Control System with Fast Reduction of Repeatable Head Position Error," U.S. Pat. No. 4,616,276, October 1986, for disk drive applications. The similarities and differences between AFC and Workman's work are discussed herein below.
After AFC was successfully implemented for the PERM disks, an AFC algorithm was tested for production disk drives. Various modifications and ideas for code optimization were added for actual implementation in order to avoid the need for extra hardware. The AFC algorithm performed well in Seagate ST9630 and ST9810 drives.
As disk drive companies, such as Seagate, plan to enter into a removable-media drive market, the performance of the removable drives will be heavily dependent on a repeatable runout (RRO) compensator as well as on a drive servo controller. Even for fixed disk drives, a powerful RRO compensator will be needed as track density increases. It is expected that a drive with more than 8,000 TPI will require the RRO compensator. Multi-platter disk drives sometimes show excessive runouts when they are installed (or tilted) in the vertical position. Even for a crashed drive due to disk slippage after an excessive shock, an effective RRO compensator will allow the drive to be used to recover user data. In summary, the RRO compensator is critical in the following applications: 1) Removable drives (i.e., PERM removable drives); 2) High-TPI drives; 3) Multi-platter drives; and 4) Data recovery operations after disk slippage.